1. Field of the Invention
The present invention relates to a method of manufacturing transparent ferrite crystals with garnet structure for us in magnetooptical elements such as optical isolators, optical magnetic field sensors, and the like, a method of producing ferrite powders preferably used therefor.
2. Related Art
The ferrite crystals with garnet structure have a composition of Y: 37.5 mol % and Fe: 62.5 mol % in which a part of Y or Fe may be substituted by other elements if necessary. As for the ferrite crystals with garnet structure for use in magnetooptical elements, ferrite single crystal bodies and ferrite polycrystal bodies were used.
Up to this time, there have been known various methods o manufacturing ferrite single crystal bodies in which a mother single crystal body was connected to a polycrystal body and a heat treatment was performed thereto to grow a ferrite single crystal into a ferrite polycrystal body, as shown in Japanese Patent Publication No. 61-1391 and in Japanese Patent Laid-Open Publication No. 63-35490.
However, in the single crystal manufacturing method mentioned above, if an amount of Fe (when a part of Fe is substituted by other elements, the other element is converted into Fe amount) is in less of an object amount of 62.5 mol %, there is a drawback such that a growth of single crystal is not promoted at all. Further, an in Fe rich state, second phases remain in the single crystal. Therefore, according to the method mentioned above, a stable single crystal growth can not be performed.
Also, in the field of manufacturing ferrite polycrystals of garnet structure, there has been known a technique for making polycrystal bodies transparent, as shown in Japanese Patent Laid-Open Publication No. 63-163815. That is to say, in the technique mentioned above, at first Ca is added in raw materials so as to reduce pores in the polycrystal bodies and to make the light absorption coefficient thereof substantially equal to that of single crystal bodies, and then V, and the like are added in raw materials so as to neutralize valence in the polycrystal bodies.
In the ferrite manufacturing method mentioned above, a light transmissivity becomes good, but magnetooptical properties such as Faraday rotation angle and Verdet constant become bad since Ca is added. Moreover, since the number of added elements is increased, there is a drawback such that an evenness of sintered bodies is deteriorated. Further, in the polycrystal bodies, a liquid phase is easily generated therein due to an existence of Ca element, and thus single crystallizations are not promoted by the known method mentioned above.
Further, as for a method of producing ferrite powder preferably used for the ferrite crystal manufacturing method mentioned above, there has been known a powder mixing method and a wet coprecipitation method.
In the known powder mixing method, raw materials such as oxides or carbonates both having a predetermined composition are mixed, and the mixed compounds are calcined and crushed to obtain ferrite powders with garnet structure. Moreover, in the known wet co-precipitation method, base solutions are added to raw solutions of nitrate of metals or sulfate of metals to coprecipitate hydroxides, and the hydroxides are washed and filtered. Then, the filtered hydroxides are dried up, calcined and crushed to obtain ferrite powders with garnet structure.
In the known powder method, since garnets are generated due to a solid-phase reaction between raw material particles and for example YI (yttrium-iron-garnet) is generated at high temperatures more than 1200.degree. C., grain growths are generated and thus particle diameters after crushing become large. Therefore, there is a drawback such that sufficient sintering density can not be obtained. Further, in the case that ferrite garnet powders a part of which is substituted by bismuth are produced, since the bismuth component is volatilized at temperatures at which a solid-phase reaction between raw material particles occurs, there is a drawback such that garnet powders having a predetermined composition can not be obtained.
In the known wet coprecipitation method, since hydroxides are directly obtained from mixing solutions including metal ions, it is possible to mix solutions in atomic order in the case of producing YIG, and thus fine garnet powders can be obtained by calcining at temperatures more than 900.degree. C. However, when use is made of ferric solutions including ferric ion as raw materials, generated coprecipitation powders become very fine, and a particle distribution of the coprecipitation powders after calcining become very broad and thus the powders have insufficient sintering density. Therefore, there is a drawback such that it is not possible to obtain ferrite crystals with garnet structure having sufficient properties from the powders mentioned above.
Further, in the case that ferrite garnet powders a part of which is substituted by bismuth are produced, if hydroxides are generated from solutions including nitrate ion or sulfate ion of iron etc. by using the coprecipitation reaction, garnetization (the change of the hydroxide powders into the garnet phase) does not occur at low temperatures due to an absorption of these ions, and thus it is necessary to make calcining temperatures at above 900.degree. C. Therefore, when a larger amount of Y is substituted by bismuth, sulfuric acid components and hydrochloric acid components can not be completely decomposed, and thus the ferrite powders having good formability can not be obtained.